1. Field of the Invention
This invention relates to the production of coatings onto particles, and more particularly, this invention relates to the production of conformal coatings onto particulate substrates.
2. Background of the Invention
Optimization of reaction surfaces is key to more efficient chemical reactions. The more controlled the catalytic interface, the more leveraged are the effects of electron confinement.
Well-controlled reaction surfaces would increase efficiencies in catalysis, energy production and energy storage. In the lithium ion battery market alone, the global U.S. dollar sales are projected to double by 2016. These sales will be concomitant with the attainment of higher efficiencies.
Lithium ion batteries for future plug-in hybrid electric vehicles will require high working voltages, on the order of about 5 volts. The advantages of these high volt chemistries include the utilization of relatively lower cost Lithium- and Manganese-containing spinels, such as LiMn2O4, immersed in electrolyte. Manganese systems are less costly and less polluting than cobalt-containing systems.
Problems abound for electrolyte-based systems. For example, structures physically immersed into the electrolyte are inevitably attacked by the surrounding electrolyte though redox chemical reactions. This significantly shortens the battery life and, even worse, creates explosion hazards.
Also, the sought-after high voltages of advanced battery systems often exceed the electrochemical resistance of electrolyte oxidation; as such, irreversible capacity degradation occurs whereby the electrolyte is compromised. This is due mainly to the high temperatures (more than 55° C.) generated and also due to the electrodes, such as high surface area spinels, reacting with the electrolyte. Specifically, at target voltages and the accompanying temperature increases, the manganese in the spinels plate on the anodes, thereby decreasing recharge capacity.
The use of high surface area materials as reaction surfaces is widespread, particularly in catalytic sciences. However, in electrochemical applications, the aforementioned degradation of electrodes and electrolyte is problematic. A possible way to prevent this degradation is by coating the aggregate. A myriad of attempts to coat aggregate include, sol-gel methods and chemical vapor deposition. However, most of these techniques are relegated to situations where the geometries of the host objects are flat, or large particles, and particularly, when the uniformity and layer thickness of the coating layers are not concerns.
When the size of host particles is reduced to the micro- or nano-scale, physical properties of the coating layer, mainly uniformity and thickness, need to be precisely controlled in order to manipulate ion and electron transport behaviors crossing the interparticle interface. The above-mentioned state of the art coating techniques fall short of these objectives.
Generally, the smaller the aggregate, the more valuable it is as a reaction surface medium. However, particles with diameters less than about 20 micrometers (i.e. microns or μm) show strong flow instability when placed in a flowing medium, such as a fluidized bed, for mixing. This is due to strong inter-particle forces which exist among ultra-fine particles. These forces include van der Waals- and electrostatic-forces. Gas tunneling, particle bridging, and clustering during suspension are common during mixing and fluidization of ultra-fine particles. FIG. 1A is a prior art picture of three fluidization beds, demonstrating clustering (on left-hand side), particle bridging (middle), and gas tunneling (right-hand side). As a result, fluidizing gas quickly passes through the bed via the empty channels. Meanwhile the dense powder regions which comprise the interior bulk of those channel structures remain static. This “polar structure” phenomenon prevents intimate gas-solid contact, the related heat/mass transfer process, and the gas-solid phase reactions sought.
A need exists in the art for a method to economically process ultra fine particles (i.e., those particles less than about 100 microns in diameter, and more commonly less than 20 microns in diameter). The method should eliminate the need for repetitive equipment motion (such as mixing, shaking or agitating) so as to effectively save energy, thereby allowing for an economical process scale-up. Also the method should assure the formation of conformal coatings to the entire surface of each of the particles.